ACCUMULATED OTHER COMPREHENSIVE LOSS (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Amount reclassified out of AOCI
Operating and maintenance expense	$ (295.1)	$ (259.3)
Nonregulated revenues	554.4	276.9
Interest expense	(29.3)	(30.4)
Income before taxes	291.8	146.2
Tax expense	(109.6)	(47.4)
Net income attributed to common shareholders	187.5	98.9
Total reclassifications	(1.5)
Amount Reclassified
Amount reclassified out of AOCI
Total reclassifications	(1.5)
Losses on cash flow hedges
Amount reclassified out of AOCI
Total reclassifications	(0.9)
Losses on cash flow hedges | Amount Reclassified
Amount reclassified out of AOCI
Income before taxes	1.5
Tax expense	0.6
Net income attributed to common shareholders	0.9
Losses on cash flow hedges | Amount Reclassified | Utility commodity derivative contracts
Amount reclassified out of AOCI
Operating and maintenance expense	0.2
Losses on cash flow hedges | Amount Reclassified | Non-regulated commodity derivative contracts
Amount reclassified out of AOCI
Nonregulated revenues	1.1
Losses on cash flow hedges | Amount Reclassified | Interest rate hedges
Amount reclassified out of AOCI
Interest expense	0.2
Defined benefit pension plans
Amount reclassified out of AOCI
Total reclassifications	(0.6)
Defined benefit pension plans | Amount Reclassified
Amount reclassified out of AOCI
Amortization of prior service costs	(0.1)
Amortization of actuarial losses	1.1
Income before taxes	1.0
Tax expense	0.4
Net income attributed to common shareholders	$ 0.6